Convert all backend function to exception

Instead of returning (False, msg) from rpc endpoints, we raise always
exceptions (the non-endpoint, internal functions can remain as is). This
means that the error paths are agnostic to how the failure is signalled
(i.e. by False, msg) and instead use only this method (exceptions) to
signal failures.

The patch also adds a log=False argument to _Fail so that trivial cases are not
logged (but usually it's a good idea to log all failures, for the record).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>